1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus equipped with a calibration function as follows. On the basis of test image data for calibration which is stored in advance in the image forming apparatus, a photosensitive material is irradiated with a light beam so as to be exposed. The photosensitive material and an image receiving material are superposed together and conveyed while contacting a heat-developing drum so as to be subjected to heat-developing processing. A test image is thereby formed on the image receiving paper. On the basis of this test image, the calibration function corrects density or color unevenness of the test image caused by non-uniformity of temperature at the surface of the heat-developing drum.
2. Description of the Related Art
Conventionally, there has been proposed an image forming apparatus by a heat-developing/transferring recording system in which a photosensitive material is irradiated with a laser beam on the basis of image data so as to be exposed. The exposed photosensitive material and an image receiving material are made to contact a heat-developing drum in a state of being superposed with each other, and are subjected to heat-developing processing. An image is thereby transferred onto the image receiving material.
With this image forming apparatus by the heat-developing/transferring recording system, there are cases in which there is non-uniformity of temperature in an axial direction of the heat-developing drum which performs the heat-developing processing. Such non-uniformity of temperature leads to density unevenness (color unevenness in a case of a color image) in the formed image. Namely, an image which should be expressed at a single density is formed on the image receiving paper at different densities which differ from each other in accordance with the position at which the image receiving paper contacts the heat-developing drum. As a result, it is impossible to obtain a high quality image. Further, there are cases in which density unevenness is caused due to changes in environmental temperature or humidity, or instrumental error, paper quality even in a laser printer, an ink jet printer, or the like.
Generally, reproducibility and stability of density are required, namely, an image whose density data is the same is desired to be formed at the same density. For this reason, calibration is of great importance in the image forming apparatus.
In order to perform calibration, test image data for calibration is usually stored in advance in the image forming apparatus.
In carrying out calibration in the image forming apparatus by the heat-developing/transferring recording system, for example, first, on the basis of the test image data, a test image which comprises a plurality of patches of different densities (colors in the case of color printing) is printed on an image recording material. The image recording material having this test image printed thereon is inserted into a calibrator which measures the density or the color of the printed test image. Further, in comparing results of this measurement with the test image data for calibration stored in advance in the apparatus, the position of the image recording material having the test image printed thereon and the position data of the test image data for calibration stored in advance (i.e., the address in the memory) are made to correspond to each other. Namely, an image writing position (the position at which a laser head which outputs a laser beam is disposed) and a processing position (where the heat-developing drum is disposed) are identified.
Thereafter, density (or color) unevenness is detected by comparing the results of measurement of the density or color, with data that corresponds to the measurement position of the test image stored in advance. Further, a correction value which offsets this density (or color) unevenness is computed. Then, during image forming processing after calibration, test image data which is stored in advance is corrected by using the computed correction value. On the basis of the corrected image data, the photosensitive material is irradiated with a light beam. Namely, the exposure amount for the photosensitive material is corrected on the basis of the position where the photosensitive material contacts the heat-developing drum during heat-developing processing, and density (or color) unevenness of an image to be formed, which unevenness is caused by non-uniformity of temperature at the surface of the heat-developing drum, is thereby corrected.
However, since the calibrator has conventionally been provided at an exterior of the image forming apparatus, each time when calibration is carried out, the calibrator must be set manually. Accordingly, there have been cases in which the position, at which the image recording material having the test image printed thereon is inserted into the calibrator, deviates from an accurate position. Further, the calibrator does not have a function of identifying the position where the image recording material having the test image printed thereon is inserted into the calibrator. Moreover, in the calibrator provided at the exterior of the image forming apparatus, there have been cases in which measurement errors are caused by the effect of external light or temperature difference inside or outside the image forming apparatus. If the image recording material having the test image printed thereon is not inserted into the calibrator at an accurate position, it is impossible to have the measurement position of the calibrator and position data of the test image data which is stored in advance correspond to each other accurately. As a result, deviation is caused between the position of measurement by the calibrator and the position where calibration is carried out (the position at which image data is exposed and which corresponds to a position where non-uniformity of temperature is caused at a surface of the heat-developing drum). Thus, highly-accurate calibration has not been realized by such a conventional image forming apparatus.
On the other hand, there has been adopted a method in which marks for detection are applied to the image recording material having the test image printed thereon so as to detect a conveying direction position of the image recording material having the test image formed thereon. However, in this case as well, since only the position, in the conveying direction, of the image recording material having the test image printed thereon has been identified, the image recording material having the test image printed thereon could only be aligned properly in an axial direction of the heat-developing drum by chance. Namely, the measurement position by the calibrator and the correction position could not correspond to each other exactly.